In a communication network consisting of nodes representing switching systems and links representing transmission facilities between pairs of nodes, the network topology, i.e., the collection of all link state information, is maintained by each node that is responsible for path computation for establishing communication between nodes. Routing protocols, which are used to select paths, may be broadly divided into two classes: link state protocols and distance vector protocols. Link state protocols require each node to maintain the states of all relevant links. Distance vector protocols require each node to maintain the state of the shortest path from itself to each relevant destination.
Link metrics are quantitative link state parameters that are associated with the quality of information transfer and are used in path computation. There are two kinds of link metrics: non-additive and additive link metrics. A non-additive link metric generically takes the form of "bandwidth". An additive link metric generically takes the form of "delay". The generic path selection problem is to determine a path according to a given routing objective, such that the bandwidth of each link on the selected path is greater than a given bandwidth threshold, and that the delay along the selected path is smaller than a given delay threshold. There are two common routing objectives in the prior art: maximum bandwidth and minimum delay.
Path metrics are derived from link metrics. For a non-additive link metric such as bandwidth, the metric value associated with a path is the minimum metric value among all the links along the path. For an additive link metric such as delay, the metric value associated with a path is the sum of the metric values of the links along the path.
The topology of the network is kept in a topology database attached to each node that is responsible for maintaining and using such information. Due to changes in the network, the topology must be updated from time to time via a topology broadcast mechanism. A topology broadcast is an event executed by a node such that a message containing link state information is advertised or distributed to all other nodes in the network.
There are two general approaches in the prior art to address topology update complexity. In one approach, the prior art reduces complexity by limiting the handling of the topology to a subset of the nodes in the network. In another approach, the prior art aggregates the link states in each subnetwork so that nodes outside the subnetwork need only maintain partial information on its topology. Typically, a representative node is elected from the nodes in the subnetwork to be responsible for aggregating and advertising the subnetwork link metrics to nodes outside the subnetwork.
The prior art that makes no compromise in the amount of link state information divides all nodes into two types: (i) master nodes with extended memory and computing capabilities, and (ii) subordinate nodes with limited memory and computing capabilities. A topology database is attached to each master node and is used by the node for path computation. The subordinate nodes rely on the master nodes for topology maintenance and path computation.
In an extreme approach for link metric aggregation, the entire subnetwork is represented by a pseudonode. Such an approach can be traced back to the IS-IS, i.e.., Intermediate System-to-Intermediate System, routing protocol. In one version of this approach, all internal link metrics of the subnetwork are reduced to a node metric for the pseudonode by taking worst cases. For an additive link metric, a popular assignment of the corresponding node metric is the "diameter" or the length of the longest shortest path between any pair of nodes in the subnetwork. For a non-additive link metric, a popular assignment of the corresponding node metric is the bandwidth of the smallest maximum-bandwidth path between any pair of nodes in the subnetwork. Another version of the approach is to let the pseudonode to represent the "middle" of the subnetwork, and assign an appropriate metric, by taking averages, to each of the links connecting the psuedonode and the exposed nodes. A variation of the above versions makes use of a diameter variance metric in addition to the diameter metric to reflect deviation from a symmetric topology.
In the OSPF, i.e., the Open Shortest Path First, routing protocol, summary link states are used for intersubnetwork routing. The summary links are essentially virtual links to external nodes. From the point of view of a subnetwork, there are internal links that connect the internal nodes together, and there are summary links that emanate from the subnetwork to external nodes. OSPF is designed to be a link state routing protocol.
In the prior art, a network is clustered into a number of levels, and each node has knowledge of only the shortest paths within its own lowest level cluster and a single shortest path to each supercluster. Path selection is based on a distance-vector method, where a routing table indicating the best next hop for each intended destination is maintained by each node.
Also in the prior art that aggregates link states, the original topology of the subnetwork is reduced to a smaller topology consisting of a subset of the nodes, namely exposed nodes, in the subnetwork. This method advertises a full-mesh virtual topology whose nodes are the exposed nodes and whose links represent paths in the original topology. The paths are determined with respect to a given routing objective. The links in the virtual topology are known as virtual links. This approach is well suited to a hierarchical routing architecture.
In other prior art, a different problem is solved. A heuristic that starts by computing a full-mesh shortest-path network consisting of a subset of the nodes in a given original network and then reducing the full-mesh network to a minimum spanning tree is used for routing to multiple destinations in a computer network, where the object is to minimize the total link weight on the multicast tree that connects the destination nodes together. There are two common spanning tree algorithms known in the prior art: maximum spanning tree algorithm and minimum spanning tree algorithm.
Although the approaches that advertise the topology in terms of a pseudonode, i.e., variations of the approach in IS-IS, offer the greatest reduction of advertised information, these approaches typically do not supply enough information for efficient routing. Moreover, the version that handles worst cases cannot reflect the link state information associated with an asymmetric subnetwork topology. The version that takes averages cannot offer any performance guarantee. The version that uses a diameter variance metric is still under investigation. When a subset of the nodes in the subnetwork are exposed, all versions do not apply.
The approach that advertises the full-mesh representation of the subnetwork requires advertisement of order M.sup.2 pieces of link state information when the number of exposed nodes is M. For a subnetwork whose nodes are not very densely connected, the amount of link state information to be advertised may well exceed that contained in the original topology of the subnetwork when M is sufficiently large. Such explosion of link state information contradicts the purpose of link metric aggregation, i.e., efficiency in routing.
The summary link approach in OSPF does not capture enough link state information for efficient routing that is subject to quality of service constraints. Specifically, it does not offer the flexibility to pick and choose link resources that best meet the quality of service requirements for a connection.
Efficient aggregation of link metrics should provide more efficient routing through the communication network. Hence, there is a need for a method for efficient aggregation of link metrics such that the number of link metrics to be advertised is minimized without appreciably compromising the information contained in the link metrics.